Monopolar lead systems are known in the field of electrical body stimulation. Typically, such systems employ a single lead having an active electrode near the distal end thereof with an indifferent electrode, often in the form of a relatively large, flat, metal plate, located on the exterior of the housing for the circuitry which develops the stimulation signal. In such a system, the electrical impulses occur between the active electrode at the distal end of the lead and the plate. The plate is sufficiently large such that the current density does not result in stimulation of the adjacent tissue.
It has been found that with an indifferent plate electrode the bulk of the charge transfer is at the periphery with little or no charge transfer at the central region. It is essentially the surface area of the periphery of the plate that establishes current density and determines whether local stimulation will occur. Thus, the formation of an indifferent electrode as a plate requires an unnecessary amount of electrode material to result in unnecessary expense, particularly if a noble metal is employed. In addition, the use of a plate requires a bonding of that plate to the surface of the housing which has, in the past, caused problems.
One approach to the elimination of the indifferent plate electrode is disclosed in U.S. Pat. No. 3,788,329, issued Jan. 29, 1974, for BODY IMPLANTABLE LEAD, in the name of Harry G. Friedman, which is co-owned with the present invention. In the referenced patent, an indifferent lead section is provided in the form of a conductor wrapped, as coils, around a portion of the length of the lead body. The coils are spaced from each other so as to increase the effective surface area. However, this type of arrangement requires two connectors, one for each of the active and indifferent electrodes. Thus, while the referenced patent does eliminate the plate indifferent electrode, it nonetheless imposes size constraints on the entire unit in that two connector assemblies must be employed.